checkout: special case error messages during noop switching

"git checkout" ran with no branch and no pathspec behaves like switching the branch to the current branch (in other words, a no-op, except that it gives a side-effect "here are the modified paths" report). But unlike "git checkout HEAD" or "git checkout main" (when you are on the 'main' branch), the user is much less conscious that they are "switching" to the current branch. This twists end-user expectation in a strange way. There are options (like "--ours") that make sense only when we are checking out paths out of either the tree-ish or out of the index. So the error message the command below gives $ git checkout --ours fatal: '--ours/theirs' cannot be used with switching branches is technically correct, but because the end-user may not even be aware of the fact that the command they are issuing is about no-op branch switching [*], they may find the error confusing. Let's refactor the code to make it easier to special case the "no-op branch switching" situation, and then customize the exact error message for "--ours/--theirs". Since it is more likely that the end-user forgot to give pathspec that is required by the option, let's make it say $ git checkout --ours fatal: '--ours/theirs' needs the paths to check out instead. Among the other options that are incompatible with branch switching, there may be some that benefit by having messages tweaked when a no-op branch switching is done, but I'll leave them as #leftoverbits material. [Footnote] * Yes, the end-users are irrational. When they did not give "--ours", they take it granted that "git checkout" gives a short status, e.g.. $ git checkout M builtin/checkout.c M t/t7201-co.sh exactly as a branch switching command. Signed-off-by: Junio C Hamano <gitster@pobox.com>